I am using the otherwise excellent app (v8.1.1) on my Mac mini (Sonoma macOS 14.1), but there is a weird bug whereby if I am viewing it in list mode, every now and then when a new track starts (maybe about every four or five tracks but I haven't counted to be sure of that), you get the main mix cutting in on top of it a few seconds later, so the two mixes are then playing at the same time. 

If you then stop playback and start again it sorts itself out, or also if you choose Home from the menu in the top left it goes back to just the Mellow Mix, but it is a bit annoying!